This invention relates to high intensity discharge lamps wherein the discharge is driven by a solenoidal electric field. More particularly, this invention relates to novel geometric structures relating the placement of the magnetic field bearing ferrites with respect to the toroidal shaped discharge tube. The primary advantage gained by the structures provided herein is the achievement of good coupling between the magnetic field within the ferrites and the plasma discharge and also the provision for a geometry in whch there is a minimum interference with the optical output of the lamp by the ferrites and in addition there are good heat sinking and cooling capabilities. Two basic geometries are described herein.
The lamps described in the invention herein are part of the class referred to as high intensity discharge lamps (HID) lamps because in their basic operation a medium to high pressure gas is caused to emit visible wavelength radiation upon excitation which is typically caused by the passage of current through an ionizable gas such as mercury vapor. The most common class of HID lamps is that in which the discharge current is caused to flow between two electrodes.
These electroded HID lamps suffer from a number of problems. First, during the operation of the lamp, there is a great tendency for the material of the electrode to be sputtered onto the surface of the lamp, blackening it and thereby reducing the optical output. Second, one of the most common causes of failure of the electroded HID lamp is the failure of the electrode itself, due to thermal and electrical stress. The life and efficiency of such tubes are therefore limited.
However, more recently, solenoidal electric field lamps have been proposed, such as the lamp described in U.S. Pat. No. 3,500,118, issued to J. M. Anderson, the same inventor herein, said patent also being assigned to the same assignee as herein. Such lamps have an electric field which is substantially circular and closes back on itself. It is along this electric field path that the discharge takes place and since it is a closed path, there is no need for electrodes and such lamps are therefore often referred to as "electrodeless lamps".
The invention herein relates to a class of lamps referred to as "high intensity discharge lamps" as noted above. Such lamps are distinguishable from ordinary fluorescent lamps in that high intensity discharge lamps typically operate at a temperature of approximately 700.degree. C. or more and at a vapor pressure of between approximately 200 Torr, and approximately 1 atmosphere. On the other hand, the low intensity discharge lamp, such as that described in U.S. Pat. No. 3,500,118, above, operates at a temperature of approximately 40.degree. C. and a vapor pressure of approximately 7 microns of Hg pressure. The high intensity discharge lamps typically consume a much greater amount of power and yield a correspondingly greater amount of optical output. The amount of power consumed depends in a positive way on the voltage drop along the discharge path. For a high intensity discharge lamp, there is a typical electric field strength of approximately 10 volts per centimeter; however, for a more standard, low intensity discharge, the electric field strength is approximately 1 volt per centimeter. This higher power rating requires a better coupling between the magnetic field within the ferrite and the electric field driven discharge. In addition, this higher power requirement also requires a certain amount of heat dissipation and cooling capability that is not required in low intensity discharge lamps. But at the same time, improvements made in the coupling and heat sinking must not appreciably interfere with the visible light output of the lamp.